Thread by @CoastalPaleo

SEA LIONS! That orca/sea lion/boat video is making the rounds right now. What are sea lions? Where did they come from? A thread about sea lion evolution! 🧵(1)

Sea lions have been recognized as fin footed carnivores along with fur seals, earless seals, and walruses for over a century; paleontologists, mammalogists, and molecular biologists have found various evidence indicating they are caniform ("dog branch") carnivores (2)

This grouping is called the Pinnipedia - pinnipeds uniquely share some features of the orbit, palate, and flippers, including elongate thumb and elongated big/pinky toes on foot. [art by P. Folkens] (3)

There have been some bizarre claims that the earless seals are more closely related to otters and weasels, and that the eared seals and walruses are more closely related; these claims are often repeated but don't stand up to rigorous analysis and won't be discussed further. (4)

So, sea lions! Two species in North Pacific and three in the southern hemisphere. California sea lions (Zalophus californianus) are the most famous - commonly in zoos and aquaria. They're smallish, only getting to 300-350kg; females 1/3 the body mass. (5)

CA sea lions are generalist fish eaters and live from southern Mexico to Alaska and are the famous sea lions at Pier 39. Two subspecies exist: the recently extinct Japanese sea lion, and the extant Galapagos sea lion. (6)

California sea lions are famous for their big head bump in males - a large mass of jaw-closing muscle on the sagittal crest of the skull, uniquely expanded in this species: (photos: U. Oregon, birdandhike.org) (7)

The species in the viral video is a female or immature male Steller's Sea Lion (NOT "stellar"), Eumetopias jubatus - these hefty pinnipeds live from the Bering Sea southwest to northern Japan and southeast to California, and eat variety of fish/squid. (8)

Eumetopias is a titanic pinniped - the largest member of the Otariidae, and the fourth largest pinniped after the two elephant seal species and the walrus, topping out at a whopping ~1000kg; average size roughly the size of largest Zalophus; females ~200-300kg (9)

Southern hemisphere sea lions Zalophus-sized or slightly larger (250-450 kg) size wise, and include Australian (Neophoca cinerea), New Zealand (Phocarctos hookeri), and South American sea lions (Otaria byronia). Here's a pair of Phocarctos I photographed in NZ: (10)

Sea lions are are distinguished by still possessing external ear flaps, already lost in earless seals (Phocidae) and walruses. They also do not have molars: their teeth are a series of little cones, sort of like dolphin teeth. They no longer chew their food like dogs do. (11)

Sea lions can also rotate their hind feet forward still - earless seals cannot, and so they wriggle about on land like a giant worm. Sea lions can actually walk... and gallop, which is a frightening sight. A surprising sight for tourists who get too close! (12)

These features, however, are all shared with fur seals - the smaller species of eared seals - and placed in the same family, Otariidae. Fur seals are generally smaller - max size as little as 64kg in Galapagos fur seal, and CA sea lion sized in Northern fur seal, ~200+KG. (13)

Fur seals thought to be less derived owing to retention of underfur - unique amongst pinnipeds, but shared with smaller species like otters that have not been marine/aquatic adapted as long; fur insulation transitions to blubber insulation in marine carnivores. (14)

Now onto fossils: many early, difficult to understand pinniped fossil discoveries originally placed into the Otariidae, including Allodesmus from California - the Desmatophocidae; desmatophocids are North Pacific, Miocene only (~20-10 Ma) (15)

The gigantic pinniped Pontolis (7-8 Ma, Miocene of Oregon) was first considered to be an otariid based on some generalized features - no doubt contributing to this Eumetopias-like reconstruction from 1909. We now know this is a gigantic sea lion-like walrus. (16)

Another similar sea lion-like walrus, Imagotaria, was originally considered to be an otariid - here's a nice "sandwich" skull of Imagotaria from the late Miocene of Santa Barbara, California: (17)

Because otariids are quite generalized, it was easy to assign poorly understood basal members of the walrus lineage or desmatophocids into this family; our understanding of pinniped evolution changed dramatically from the 1960s-90s thanks to loads of west coast USA fossils (18)

The first "old" true otariid fossil was a bit of a surprisingly scrappy one: Pithanotaria starri, what looks like a spectacular skeleton but is actually a mold of the bones that crumbled to powder upon discovery. Late Miocene, Santa Barbara, CA: (zoochat.com) (19)

In the 1960s/70s, new fossils from latest Miocene (5-8 Ma) deposits in Mexico and Baja California produced Thalassoleon - a large fur seal possibly ancestral to all fur seals, or otariids in general. More recently discovered in early Pliocene of CA and from Japan (20)

Fossils from Pliocene epoch - last period of generally warm climate before Pleistocene glaciations - has very first fossil record of extant genus - Callorhinus gilmorei, a dwarf ancestor of modern Northern Fur Seal- lived alongside strange walruses on California coast (21)

In 2013 I found this beautiful little jaw misidentified as the early dwarf walrus in a museum in Orange Co., CA - @Imagotaria and I named it Eotaria crypta in 2015: earliest known fur seal, with some protopinniped-like features, and 5-7 my older than Pithanotaria. (22)

You've noticed that nearly ALL these fossils are from California, Mexico, and Oregon: there are some great specimens from Japan, but all this seems to point to North Pacific origin. There are only a couple of 2-3 myo otariids from entirety of southern hemisphere! (23)

Surprisingly, molecular phylogeny indicates that most southern hemisphere species of otariids form a clade that crossed equator from North Pacific 2-5 mya, and rapidly diversified. This probably means that "sea lions" are not a natural biological group (clade) (24)

Oldest southern hemisphere otariid is Hydarctos lomasiensis from ?Pliocene of Peru - poorly dated, could be anywhere from 2-6 Ma. (25)

Then there's a bunch of Pleistocene scraps from Australia and South America probably belonging to modern genera or species - and Neophoca palatina from middle Pleistocene of NZ, in same genus as Aust. sea lion. (26)

Returning full circle to North Pacific - fossils of true sea lions - Zalophus and Eumetopias, from Pliocene/early Pleistocene of Japan, seem to pre-date fossils from California/Oregon, suggesting that walruses were "still in the way". (27)

The northern fur seal, Callorhinus, evolved in place along the California coast, and never went extinct - one of the only marine mammals to persist through severe faunal change and/or marine mammal extinction at the end of the Pliocene or early Pleistocene. (28)

After bizarre walruses like Dusignathus and Valenictus became extinct on the Pacific coast around 2 mya, the first true sea lion fossils show up in middle Pleistocene rocks of Oregon, like Proterozetes ulysses - probably new immigrants from Japan: (29)

Owing to low sea level & erosion, marine mammal bearing Pleistocene marine deposits are rare and generally small in outcrop/volume, so I've spent quite a bit of time prospecting some of these localities, with only scraps to show for it. (30)

Sometimes the scraps are good ones: here's a nice mandible discovered by @AshPoust from mid Pleistocene of Oregon belonging to Proterozetes, which we finally published a few years ago: (31) tandfonline.com/doi/abs/10.108…

The fossil record of otariids is not great - it's still *much better* than northern hemisphere earless seals (Phocinae) - but many more fossils, including complete skulls, are needed to answer a number of remaining questions in pinniped evolution. (32)

Many undescribed fossils exist in museum collections and await description, just like the mandible of Eotaria crypta! Here's some great new material of Pithanotaria currently under study by @VelezJuarbeJ for example: (33)

In sum: kinda crappy fossil record; murky origins; terrible fossil record from last 2 million years = lots of unknowns regarding recent evolution of extant species; difficult to interpret skeletal anatomy - historically enigmatic group. Plenty left to do! (34)